[ICOM] 756ProIII PA Failure

[Adam Farson]

Hi Frank,

Icom America Parts in Bellevue WA should be able to supply you with a
matched pair of 2SC1972's.

It is standard design practice to leave the PA unit live even when the radio
is powered down. Manufacturers do not wish to include a 30A contactor in
the radio to switch the primary DC supply rail to the PA.

As other list members have suggested, all radio equipment should be isolated
from antennas, power sources and peripheral circuits (such as phone lines)
when not in use - especially in a lightning-prone location such as Florida.

The new MOSFET PA Unit may not be compatible with the older BJT version of
the Pro3.

Cheers for now, 73,
Adam VA7OJ/AB4OJ


-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of Frank Haas KB4T
Sent: 25-Aug-08 05:20
To: icom at mailman.qth.net
Subject: [ICOM] 756ProIII PA Failure


This past weekend the output power on my 756ProIII dropped to 3 watts while
I was calling CQ on CW. Upon investigation I found that both of the 2SC1972
driver transistors were shorted base to collector. One of the two drivers
had gotten so hot it charred the circuit board holding
D1 in place over it. I don't see any other problems. 

There is no obvious cause for this failure. No power line surge or lightning
at the time of the failure. The PS-125 power supply seems to be delivering a
stable 14V to the radio. Nearly one year ago to the day my home took a
lightning strike which damaged other gear but didn't seem to affect the
ProIII. I suppose it's possible that this is a long delayed stress failure
from that incident. The ProIII was running 100 watts output to a tuner and a
ladder line fed 80M full wave loop that has no obvious problems. 

With the ProIII down, I moved my backup ProII into the primary operating
position and I'm still on the air without any problems now.

It appears that 2SC1972 transistors are hard to find now. I read on Adam's
web site that there is a later version of the PA available using newer style
components. 

My question is: Should I try to locate a pair of 2SC1972s and complete the
repair for about $150 or replace the PA? 

I was surprised to find that power is applied to the PA all the time even
with the rig turned off. I wonder if this is a good practice?

73,
 
Frank N. Haas KB4T
Florida